1. Field of the Invention
This invention relates to the electrostatic transfer of toner images in electrostatographic recording apparatus. More specifically, it relates to an image forming apparatus and method in which the transfer of a toner image to a receiving surface, either of a receiving sheet or an intermediate member, is controlled in varying ambient conditions.
2. Description of the Prior Art
In the electrostatic transfer of xerographic images to paper or another receiving medium, the toner image is transferred to the paper or other receiving medium from the toner image bearing medium by electrical attraction. In a typical electrostatographic recording apparatus, the paper is supported on a transfer roller (or belt) as it is fed into a transfer nip between the transfer roller and the toner image bearing medium. A DC potential of about 250 to 3000 volts is applied between the transfer roller and the toner image bearing medium. Light pressure is applied in the nip to hold the paper in close contact with the toner image bearing medium.
U.S. Pat. No. 3,781,105 to Meagher, issued Dec. 25, 1973, suggests a multilayered transfer backing roller having intermediate conductivity. More recently, typical backing members having a polyurethane blanket with a resistivity of about 0.5.times.10.sup.9 to about 5.0.times.10.sup.9 ohm-cm at 70.degree. F. and 50 percent relative humidity are used to provide an adequate system time constant while minimizing the amount of pre-nip ionization that is generated. However, the resistivity of intermediate conductivity materials is not stable over a typical range of operating temperatures and humidities and also changes somewhat as the roller ages. Problems associated with humidity are known to result in image quality degradation in the form of ionization defects and reduced transfer efficiency. Although somewhat different in degree, these effects exist both with conventional transfer of a toner image directly to a receiving sheet backed by a transfer member of intermediate conductivity and with transfer of a toner image directly to a transfer member that functions as an intermediate transfer member and which then transfers the toner image to a receiver sheet. Although rollers are typically used as both backing members and intermediate transfer members, belts and other configurations for such transfer members are also used.
U.S. Pat. No. 5,084,737 to Hagen et al also notes that an increase in humidity causes moisture to be absorbed by polyurethane or other material substance being used for the intermediate conductivity layer (blanket) on a transfer roller. A reduction in resistivity in the transfer roller can impact adversely on transfer efficiency. To overcome this problem, the prior art employs a constant current power source to create the transfer field. The use of a constant current for transfer thus reduces the affect of changing resistances of the transfer member and paper which arise because of changing humidity in the apparatus. See, for example, U.S. Pat. No. 5,036,360 to J. F. Paxon et al, issued Jul. 30, 1991.
U.S. Pat. No. 5,568,228 to Tombs discloses an electrophotographic image forming apparatus that includes a transfer station having an erasing radiation source. Variations in resistance of the transfer member due to relative humidity, age or the like or of the receiving sheet are compensated for by adjusting the irradiation source to maintain consistency in the response time of the transfer field.
In the apparatus described by Tombs, changes in transfer member resistivity are immediately compensated for by including the erase source in a process control algorithm. The process control includes a device that measures a response which correlates reasonably with changes in transfer member resistance or blanket resistivity. Tombs notes that either direct measure of transfer member resistance or indirect measures may be used. Indirect measures include measuring the density of a transfer control patch or its residual. Similarly, the relative humidity change can be directly measured and input. The adjustment of the erasing radiation can be made during cycle up or between images, depending on the need to adjust for rapidly changing conditions. In measuring the density of a transfer control patch, a toner patch is formed on the primary image forming member such as the photoconductor and transferred to the transfer drum. The density of the patch transferred to the transfer drum can then be measured. Alternatively, according to Tombs density of the untransferred residual toner remaining on the primary image forming member may be measured and used as a basis for adjusting erasing irradiation.
The need to transfer a patch to the transfer roller is undesirable since this requires cleaning of the transfer roller or may cause transfer to the back of a next succeeding receiver sheet. Additionally, a densitometer or reflectometer is required to sense density of the transferred patch.